This invention relates to optical fiber communication systems of the type using incoherent light sources, such as light-emitting diodes, commonly called LEDs.
Such diodes may be directly modulated and have been found attractive for early versions of optical communication systems, particularly those using multimode fibers. The most attractive multimode fiber for use in such systems presently appears to be the parabolic-index-profile fiber, that is, a fiber in which the core is graded in index of refraction from a maximum index on its central axis parabolically down to a minimum index adjacent to the cladding.
There have been a number of studies of ways of obtaining effective systems using LEDs. In general, in the prior art, it is assumed that the radius of an LED should be approximately equal to the radius of the core of the fiber, whether it be a step-index-profile fiber or a graded-index-profile fiber. For example, see the copending patent application of E. A. J. Marcatili, Ser. No. 571,103, filed April 24, 1975, and assigned to the assignee hereof. Moreover, when other relationships have been investigated as to diode size, it has been assumed that, when the diode radius is smaller than the fiber core radius, additional optics will inevitably be required. An example of such an article is that by K. H. Yang et al, Applied Optics, Volume 14, page 288 (February 1975).
Nevertheless, that conclusion may be incorrect; and, in any event, it is highly undesirable for an economical commercial system to be required to use such additional optics, which will tend to be relatively expensive compared to the rest of the system.
The nature of the relationships which may be obtained with a parabolic-index fiber when such additional optics are lacking has not been sufficiently investigated. Substantial advantage may be realized by pursuing these questions to their ultimate conclusion.
In particular, it is an object of the present invention to obtain a more effective and more efficient optical fiber communication system utilizing incoherent light. It is also desired to have the diode and a parabolic-index fiber directly bonded together.